Gates of Fury by Tamar Sloan

Another Sin. Another Gate. But this time, the Grail is within reach.
For Arielle and Reign, the obstacles seem insurmountable. Mercy City is ravaged. Demons are wreaking havoc as they determinedly seek to open the next Gate of Hell. And deeply buried truths can no longer be denied.
But hope has never been more alive. Despite the odds, Arielle and Reign’s relationship is blooming. And they’ve learned of a magical chalice. The Holy Grail is the salvation they’ve been seeking all along.
As fragile alliances are formed and old friends return, the possibility of victory is within reach. This Innocent can be saved, evil can be stopped, and the Gates closed forever. Although one lesson must still be learned…
In the quest for the Grail, nothing is as it seems.